Rotors with permanent magnets consist of a cylindrical core made of solid or rolled iron, around which there is disposed a plurality of magnets usually made of strontium or barium ferrite, or samarium cobalt and generally in the form of arcuated plates having internal surfaces with a contour that coincides with the circumferential contour of the rotor to which said magnets are mounted.
During operation, the rotors with magnets are submitted to centrifugal forces, moments of force and stresses of thermal origin, all of very high intensity. Due to these forces, the retention of the magnets to the rotor assumes a great importance in determining the efficiency of the motor. Different methods and techniques of retaining the magnets to the rotor are known in the art, such as that disclosed in U.S. Pat. No. 5,175,461 (Ziegler). Among the usual techniques for retaining the magnets to the rotor, the following are known: gluing said magnets to the external surface of the rotor; compressively surrounding the magnets close to the rotor core by an external cover, using or not using glue for the previous fixation of the magnets to the core; and enclosing the magnets in a structure, such as a cage mounted around the core, where the magnets are disposed in predetermined positions in said structure.
Except for the magnet enclosure technique, the other techniques mentioned above do not allow to obtain a rotor which guarantees a high reliability and which maintains unaltered a determined performance during the useful life of the motor. The main failures that may occur with the rotors constructed according to these techniques are: degradation of the magnets, due to the appearance of cracks, chips, etc.; degradation of the glue used to affix said magnets to the core, with loss of its properties and even of its function, which may generate residues; and degradation of the external cover that surrounds the magnets, with permanent deformations and loss of its function, in case it has a pre-tensive function, which may impair the air gap between the rotor and the stator. The use of covers made of a thin material, usually iron, steel, brass, copper, aluminum, etc., also presents problems of electrical losses which, in certain applications, may be very high. Moreover, the covers, when subjected to continuous and extended variations of temperature and rotation, due to the operational regimen of the equipment where the rotor is mounted, may suffer irreversible deformations.
While the magnets may support relatively high compression forces, they are fragile regarding tension and impact. The prior art solutions to aggregate the magnets to the core imply both in compression and tension forces, though shearing forces may also occur in a less significant form, resulting from torque and accelerations.
Among the stresses that affect both the magnets and the cover, those of thermal origin are usually much more significant than the other stresses supported by said elements during the operation of the motor to which they are aggregated. However, the thermal stresses depend on the manner used to maintain the magnets retained to the rotor and on the variation of the temperature to which the motor is submitted.
The origin of the thermal stresses lies on the fact that the coefficients of thermal dilatation of the material of which the magnet is made and of the material of the core, cover and glue are different from each other. The difference in the coefficients of thermal dilatation implies in the tendency of occurring relative displacements between said elements, which may impair the physical integrity of the magnets, mainly when the fixation thereof to the core is obtained by gluing or by the provision of a cover retaining them close to the core.
Besides the difference of the coefficient of thermal dilatation between the materials that form the above cited elements, the magnets have, due to their constructive form and magnetic orientation, differentiated coefficients of thermal dilatation, which vary according to their dimensions in the radial, transversal and longitudinal directions.
Though the solution using the enclosure of magnets solves the above cited problems for the glue and cover solutions, it has other inconveniences, such as high cost and reduction of motor efficiency, due to the large air gap provided in function of the large design dimensions required, since the materials having low losses are structurally fragile or have high cost.
Due to the inconveniences described above, said known prior art solutions are not recommended to be used in equipments requiring low cost, low generation of residues and high reliability, such as it occurs, for instance, with the hermetic compressors for refrigerating systems.